In the manufacturing of cans having a three-piece construction, a thin walled metal sheet is rolled into a cylinder to create the body portion of the container. The body portion is rolled such that the two longitudinal edges of the sheet overlap. A pair of welding electrodes are used to seam weld the overlapping edge portions to create an open ended cylinder. Metal ends are affixed to the two open ends of the cylinder (e.g., via a seaming operation) to complete the production of the can.
Many three-piece cans are used to contain various types of food products. As such, the interior surfaces of the can are coated with a lacquer or some other surface coating to prevent the food products from contacting the metal surfaces of the can. During production of the cans, the lacquer is applied to the metal surface of the container-body prior to the above-described rolling and seam welding operations. In this process of applying the lacquer, all but the overlapping edge portions of the body of the container are coated. The edge portions are not coated with lacquer in order to provide a metal to metal interface for the welding operation.
During the application of the lacquer to the body portion of the container, lacquer may be inadvertently dripped onto or other surface contaminants may be present on the two longitudinal edge portions. When the body portion of the container is rolled into a cylinder, these surface contaminants may remain on the surfaces to be welded or, more specifically, they may become sandwiched between the overlapping edge portions. The surface contamination may create a potential hazard when the two longitudinal edge portions are welded because the contaminated areas increase the electrical resistance encountered by the welding electrodes.
Some seam welding systems employ a control system which attempts to provide a constant current to the welding electrodes during the welding process which is affected by varying the voltage. As such, the welding voltage may vary according to the electrical resistance encountered by the welding electrodes as they advance along the overlapping longitudinal edge portions of the container body. If an increased resistance is encountered during welding, such as due to the presence of a surface contaminant in the weld area, the voltage applied to the welding tip is increased in order to maintain a constant current. Conversely, if a decreased resistance is encountered during welding, the voltage applied to the welding tip is decreased to attempt to maintain a constant current. These control systems monitor the current, and when the current drops due to an increased resistance, the voltage is increased in an attempt to maintain a constant current for welding. Once the welding electrodes move off of the area of increased resistance, the electrodes encounter an area of lower resistance where the current then increases. Since the known control systems monitor the current, they are unable to detect the need to lower the voltage, and thereby the current before advancing beyond the contaminated area. As such, when the system does reduce the voltage to return the current to the desired level, the welding electrode has already received a potentially high current. In some cases, this dramatic increase in current may causes damage to the welding electrodes. If the welding electrode is damaged, the manufacturing line must be stopped in order to replace or repair the electrodes. In the manufacturing of cans, it is undesirable to stop the manufacturing line because high costs are incurred due to losses in productivity.